Reflected light and inverse square law

A “perfect” theoretical mirror allows you to increase the distance the light travels and thus get a slower fall-off near the subject.

But nothing in life is perfect, if the mirror is dusty and some light is bounced off of the dust, then that light in essence “originates” from that point and the fall-off rate is calculated from there.  So what results is a mix of light, some from the original source reflected off of the clean parts of the mirror, and other light reflected off of the dust of the mirror. I suspect that the resulting fall-off rate is also a mix.

It’s a similar issue to the sun shining through a dusty window, some rays have long fall-off rate while others have a faster rate starting from the window as the source, not the sun as the source, so it’s sort of a mix. At least, that’s my theory…

In the March 2023 issue of American Cinematographer, there’s a really good test and article about the inverse square law in the Shot Craft section. I’d recommend checking it out if you haven’t seen it yet.